1. Field of the Disclosure
The present disclosure relates to a photocathode semiconductor that improves generation of mono-energetic electrons, quantum efficiency and its lifetime by using a superlattice structure. The photocathode semiconductor device is a suitable electron source for high brightness and long lifetime.
2. Description of the Related Art
Conventionally, various techniques are proposed to apply photocathode semiconductor devices as electron sources for accelerators, electronic microscopes, inverse photoelectron spectroscopy or other. The principle of photocathode semiconductor devices is based on a photo electron emission phenomenon that occurs when a semiconductor material is irradiated with a laser-light.
One such technique is disclosed in Japanese Patent No. 3154569 and Japanese Patent No. 2606131 identified below.    http://www.semi.te.chiba-u.jp/mqw.htm. (Yoshikawa laboratory, Department of Electronic and Mechanical Engineering, Faculty of Engineering, Chiba University; the page is as of May, 2008) discloses a technique directed to a superlattice structure, also called as a multi-quantum-well structure.
A quantum-well structure is formed by using two or more material with different band gaps or different doping concentration so that a material is held between layers of another material that has the potential offset in the conduction band or the valence band.
The quantum well structure has one layer called “well layer” that has a small band gap and by which electrons and holes are confined, and another layer called “barrier layer” with a wide band gap that serves as a barrier for the carriers.
A multi-quantum-well structure refers to one type of quantum well structure with multiply provided well layers, distinguished from that which has a singular well layer, called a single quantum well structure.
As for the energy levels of electron, energy bands as observed in a common semiconductor material are also seen in the quantum well structure. In addition to the normal energy bands, the quantum well structure produces discrete energy “sub bands” or “mini bands” in a conduction band or a valence band thereof.
Electrons can transit between those mini bands.
Japanese Patent No. 3154569 discloses quantum efficiency improvement without degradation of polarization in a polarized electron source. This is achieved by providing a semiconductor multilayer mirror underneath an undersurface of a second semiconductor (strained GaAs semiconductor) layer of the electron source. The second semiconductor emits polarized electrons when an excited laser is applied thereto and the multilayer mirror causes multiple reflection of the excited laser between itself and the top surface of the second semiconductor. This yields increase of amount of light energy absorption in the second semiconductor layer without necessity of increasing the thickness of the second semiconductor.
Japanese Patent No. 2606131 forth an objective of achieving an excellent compromise between a high degree of spin polarization and high quantum efficiency in a semiconductor spin polarization electron source, and discloses providing the following elements on a substrate: A block layer having an electron affinity lower than the substrate and having a thickness of equal to or less than the electron wave length. As a region to produce spin polarized electrons a p-type conductive, a short-period strained superlattice structure that does not cause lattice relaxation and comprises a strained well layer having a lattice constant larger than that of the substrate and a thickness of less than the electron wave length, and a barrier layer having a lower energy of valence band than the strained well layer. A surface layer absorbing band bending.
In this structure, by receiving a compressive stress, the strained quantum well layer produces a further wide gap of energies between a band of heavy holes and a band of light holes, which occur in a valence band of the superlattice structure.
Accordingly, realization of a photocathode semiconductor device having a capacity against a large current needed to generate high brightness electron beam based on the use of a superlattice structure, is eagerly sought.
The present disclosure is made in view of the above, and its objective is providing a photocathode semiconductor device having a capacity against a large current to generate high brightness electron beam by using a superlattice structure.